Although the present background describes the functionality and limitations of synthetic aperture radar systems or a particular class of communications, such description is merely provided to exemplify a problem capable of resolution with the present invention. Any discussion herein directed to specific radars or communications protocols should not be taken by those skilled in the art as a limitation on the applicability of the invention described herein.
Radar systems use time delay measurements between a transmitted signal and its echo to calculate range to a target. Ranges that change with time cause a Doppler offset in phase and frequency of the echo. Consequently, the closing velocity between target and radar can be measured by measuring the Doppler offset of the echo. The closing velocity is also known as radial velocity, or line-of-sight velocity. Doppler frequency is measured in a pulse-Doppler radar as a linear phase shift over a set of radar pulses during some Coherent Processing Interval (CPI).
Radars that detect and measure target velocity are known as Moving-Target-Indicator (MTI) radars. MTI radars that are operated from aircraft are often described as Airborne-MTI (AMTI) radars. When AMTI radars are used to detect and measure ground-based moving-target vehicles, they are often described as Ground-MTI (GMTI) radars.
In MTI radars, the angular direction of a target is presumed to be in the direction to which the antenna is pointed. Consequently, a MTI radar generally offers fairly complete position information (angular direction and range) to some degree of precision, but incomplete velocity information since Doppler is proportional to the time-rate-of-change of range, i.e. radial velocity. Tangential velocities, that is, velocities normal to the range direction do not cause a Doppler shift, so are not measured directly. Tangential velocities can be measured indirectly by tracking the angular position change with time, but this requires a somewhat extended viewing time for any degree of accuracy and/or precision.
Multiple MTI systems might be employed in concert, each measuring radial velocities in different spatial directions. In this manner, a two-dimensional (or even full three-dimensional) target velocity vector may be estimated. This technique, however, requires that the radars be widely separated to facilitate the necessary triangulation (e.g., being based on different aircraft in the case of GMTI systems).
GMTI systems are often employed from moving radar platforms such as aircraft, that is, the radar itself is in motion with respect to the ground. Consequently, the stationary ground itself offers Doppler frequency shifts. In addition, since different areas of the ground are within view of different parts of the antenna beam, and have somewhat different radial velocities, the ground offers a spectrum of Doppler frequencies to the radar. This is often referred to as the clutter spectrum, and can mask the Doppler returns for slow-moving target vehicles of interest. Of course, if a target's Doppler is outside of the clutter spectrum, its detection and measurement are relatively easy. This is called “exoclutter” GMTI operation. Detecting and measuring echo responses from slow-moving target vehicles that are masked by the clutter are considerably more difficult, and are called “endoclutter” GMTI operation.
The ability to observe targets masked by clutter is often called “sub-clutter visibility.” Reducing the effects of clutter on detecting and measuring such targets' motion is often termed “clutter suppression.” This is most often accomplished by employing multiple antennas on a single aircraft arrayed along the flight direction of the radar, and is often called a Displaced Phase Center Antenna (DPCA) technique, or Interferometric GMTI. The following patents provide background information on the use of more than one antenna in radar systems: U.S. Pat. No. 4,885,590, issued Dec. 5, 1989 to M. A. Hasan, entitled “Blind speed elimination for dual displaced phase center antenna radar processor mounted on a moving platform”; U.S. Pat. No. 4,086,590, issued Apr. 25, 1978 to W. B. Goggins, entitled “Method and apparatus for improving the slowly moving target detection capability of an AMTI synthetic aperture radar”; U.S. Pat. No. 5,559,516, issued Sep. 24, 1996 to J. A. Didomizio, R, A. Guarino, entitled “Dual cancellation interferometric AMTI radar”; U.S. Pat. No. 5,559,518, issued Sep. 24, 1996 to J. A. DiDomizio, entitled “Low target velocity interferometric AMTI radar”; and U.S. Pat. No. 5,818,383, issued Oct. 6, 1998 to E. F. Stockburger, H. D. Holt Jr., D. N. Held, R. A. Guarino, entitled “Interferometric moving vehicle imaging apparatus and method.”
Interferometric techniques allow making independent angle measurements not affected by target motion, thereby facilitating discrimination of a moving vehicle in one part of the antenna beam from clutter in another part of the antenna beam that otherwise exhibits identical Doppler signatures. Interferometers can be constructed from separate distinct antennas, or from monopulse antennas that offer the equivalent of separate distinct antenna phase centers in a single structure. Although interferometric systems and method provide improved target analysis through clutter reduction, the acquisition of tangential velocity measurements within a single CPI still remains problematic, and has not been adequately addressed in the art. There remains a need for a more complete target velocity vector measurements and analysis for time-critical moving vehicles. Furthermore, there is a need that such measurement and analysis continue to be provided from the vantage point of a single system.